DNA, isolated from temperate coliphage lambda-h80dlacps and containing the sequence coding for beta-galactosidase, was used to initiate in vitro synthesis of the enzyme in the presence and absence of various carcinogens, especially alkylating agents. Beta-galactosidase activity was measured colorimetrically by hydrolysis of O-nitrophenyl-beta-D-galactopyranoside. Inhibition of synthesis of enzyme activity resulting from carcinogen interaction with DNA and with other components of the system is measured. Repair of DNA will be studied by pretreating portions of damaged DNA with enzyme mixtures postulated to effect repair excision and synthesis, and then using repaired DNA as template for beta-galactosidase synthesis.